This invention relates to a discharge lamp lighting device including an interlocking mechanism, and more particularly to a technique which can effectively be applied to such a small-sized discharge lamp as a metal halide lamp which can be attached by means of threading engagement to a lamp socket.
In general, a discharge lamp (hereinafter sometimes simply referred to as the "lamp") such as a mercury lamp, a sodium lamp, or a metal halide lamp is high in luminous efficiency. On the other hand, it has such handling disadvantages of the lamp that the work for attaching the lamp is troublesome, and the like. In order to make it easier to handle the lamp, a discharge lamp is developed which can be attached by means of threading engagement to a lamp socket as in an incandescent lamp (see Japanese Patent Unexamined Publication (KOKAI) No. 6301/81).
Another disadvantage, with which the above conventional lamp is encountered, is that there is a need of a provision of a start circuit for starting the lighting of the lamp. The worse is that there is a need of a provision of such a high voltage as, for example, 3.5 to 5 kV. In consideration of the high voltage for starting, a light device for a discharge lamp is usually provided with a safety measure (so-called interlocking mechanism) for preventing an occurrence of high voltage when the lamp is not yet attached to the lamp socket. In the conventional interlocking mechanism, there is a provision of a switch capable of performing a switching operation depending on whether the lamp is attached or not, so that the start circuit will not be actuated when the lamp is not attached, based on the operation of the switch (see Japanese Patent Unexamined Publication (KOKAI) No. 298190/96, or Japanese Utility Model Unexamined Publication (KOKAI) No. 13270/86).
The inventor of the present invention paid attention to the convenience of a discharge lamp which can be attached by means of threading engagement and attempted to develop an interlocking mechanism suitable thereto. As a result of study, it was revealed that there are still several problems to be solved.
The first problem to be solved is that since the power source is not cut off in the method for prohibiting actuation of the start circuit when the lamp is not attached, there is a fear that when the circuit should be subjected to failure by one reason or other, a high voltage would be generated. Therefore, a method capable of more assuredly preventing actuation of the start circuit is demanded.
The second problem is that although the attachment of the lamp by means of threading engagement is performed in the same simple manner as in an incandescent lamp, the method of attachment by one touch is impossible to employ because a screwing-in operation is required. For this reason, there is a possibility that the operator is surprised by sudden lighting of the lamp during the attachment work. Since not only the emission of light but also the abnormal noises due to discharge break-down occur when the lamp is lit, there is also a fear that the operator unconsciously releases his/her hold of the lamp with a result that the lamp is broken.
The third problem is that a spark tends to occur between the lamp and a high contact member on the side of the lamp socket during the attachment work of the lamp by means of threading engagement and as a result, those areas are locally damaged. Although the second and the third problems can be obviated by performing the attachment work of the lamp in the OFF-state of the prime switch of a power source, this cannot be a good solution of the problems because other lamps are also required to be in the OFF-states.